Ion Heating Experiments and Improvement of Ion Heat Transport in LHD

Ion heating experiments have been intensively carried out in high- and low-Z_eff conditions of Large Helical Device plasmas. In high-Z_eff plasmas utilizing neon or argon gus puffing, the ion heating power normalized by ion density (P_i /n_i) increases with Z_effL and the central ion temperature increases with P_i /n_i without saturation. The central ion temperature of 13.5 kV was achieved in an argon-seeded plasma, strongly suggesting the capability of the helical configuration to confine high-performance plasmas. In low-Z_eff experiments, improvement of ion heat transport was realized in the core plasmas heated by high-power neutral beam injections. The ion temperature has a peaked profile with steep gradient in the core region (ion internal transport barrier). The transport analysis indicates that the anomalous transport is reduced in the core region, where the negative radial electric field is predicted by the neoclassical ambipolarity. Improvement of ion heat transport with positive radial electric field was also successfully demonstrated utilizing strongly focused electron cyclotron resonant heating, suggesting further improvement of ion heat transport in reactor-relevant plasmas.